In many computer systems, the storage and retrieval of information for and by computer applications is handled by one or more central storage systems. For example, one type of storage system commonly used in personal computers is a file-folder-and-directory-based system, also termed a “file system.” Such file systems organize pluralities of files into hierarchies to create an abstraction of the physical organization of the storage medium used to store the files. Generally, such an organization into a hierarchy occurs at the operating system level. The files stored generally include the file hierarchy itself (the “directory”) embodied in a special file maintained by the file system. This directory, in turn, maintains a list of entries corresponding to all of the other files in the directory and the nodal location of such files in the hierarchy (herein referred to as the folders).
The use of file system for central storage has several limitations. These may be overcome by using relational database technology as the underpinning of a central storage system.
In an application using data stored in a storage system, some activity of the application may be predicated on changes to the data stored in the storage system. Based on changes to individual objects or groups of objects in the store, applications may want execute relevant business logic or display logic. For example, an email program may wish to know when a new email message arrives. In certain systems, email messages arrive and are stored on the storage system. When a new message arrives, the email program can then perform any internal processing using the fact that a new email message has arrived or using the content of the email message. For example, some email programs contain rules used to process messages. The application of these rules is triggered, in some cases, by the arrival of a new message. Additionally, there may be user interface changes based on changes to data stored in the storage system. For example, upon the arrival of a new message, an alert may be presented to the user which indicates that a new message has arrived. A list of messages may be refreshed to include the new message.
The type of data change that an application may be interested include creation of objects, deletion of objects, updates to objects including moving an object or security changes to objects. The underlying relational database may not provide triggers which can be used to provide these updates.
Certain components require continuous monitoring of changes to objects stored in the relational database. For example, anti-virus programs require that changes to objects of certain types (including addition of such objects) trigger virus scans. Similarly, full-text indexing of entities is done on a continuous basis. If there is an interruption in the connection of an application to the underlying relational database, notifications may be lost and, upon resumption, it is possible that much work will be required to resume operations. Seamless resumption of monitoring (and responding) to changes, including changes occurring during the interruption of a connection of an application to the relational database, would allow savings in processing and time when the connection is resumed.
In view of the foregoing, there is a need for a change notifier that can be used in a database-based storage system and provides for good performance and scalability. The present invention satisfies this need.